It is known a surface acoustic wave device functioning as a filter device or oscillator used in mobile phones or the like and an acoustic wave device such as lamb wave device or film bulk acoustic resonator (FBAR) using a piezoelectric thin film. As such acoustic wave device, it is known a device produced by adhering a supporting body and a piezoelectric substrate propagating a surface acoustic wave and by providing interdigitated electrodes capable of oscillating the surface acoustic wave on a surface of the piezoelectric substrate. By adhering the supporting body whose thermal expansion coefficient is lower than that of the piezoelectric substrate onto the piezoelectric substrate, the change of a size of the piezoelectric substrate responsive to temperature change is reduced so that the change of the frequency characteristics as the surface acoustic wave device is reduced.
For example, it is proposed, in patent document 1, a surface acoustic wave device having the structure produced by adhering a piezoelectric substrate and silicon substrate with an adhesive layer composed of an epoxy adhering agent.
Here, it is known that, in bonding a piezoelectric substrate and silicon substrate, a silicon oxide film is formed on a surface of the piezoelectric substrate, and a silicon substrate and the piezoelectric substrate are bonded through the silicon oxide film (patent document 2). In the bonding, plasma beam is irradiated onto the surfaces of the silicon oxide film and silicon substrate to activate the surfaces, followed by the direct boding (plasma activation method).
Further, it is known that a surface of the piezoelectric substrate is made a roughened surface, a filler layer is provided on the roughened surface to provide a flattened surface, and the filler layer is adhered onto a silicon substrate through an adhering layer (patent document 3). According to this method, an epoxy based or acryl based resin is used for the filler layer and adhering layer, and the bonding surface of the piezoelectric substrate is made the roughened surface to reduce the reflection of bulk wave and to reduce spurious wave.
Further, it is known direct bonding method of so-called FAB (Fast Atom Beam) system (patent document 4). According to this method, neutralized atomic beam is irradiated onto the respective bonding surfaces at ambient temperature to activate them, followed by direct bonding.
On the other hand, according to patent document 5, it is described that a piezoelectric single crystal substrate is directly bonded to a supporting body made of a ceramic (alumina, aluminum nitride, silicon nitride) and not to a silicon substrate, through an intermediate layer. A material of the intermediate layer is silicon, silicon oxide, silicon nitride or aluminum nitride.
On the other hand, according to a composite substrate described in patent document 6, in the case that a piezoelectric substrate and a supporting body are bonded through an organic adhesive layer, Rt (maximum cross sectional height of roughness curve) of a bonding surface of the supporting body to the piezoelectric substrate is made 5 nm or larger and 50 nm or smaller, so as to obtain the effect of preventing cracks by relaxing of a stress.
Further, according to patent document 3, in a surface acoustic wave device produced by adhering a piezoelectric substrate and a supporting body through an adhesive layer, unevenness is formed on a bonding surface of the piezoelectric substrate, a filling agent is applied on the bonding surface to form a filler layer, and the filler layer and supporting body are adhered. In this case, micro unevenness is provided on the bonding surface of the piezoelectric substrate to make the arithmetic average roughness 0.1 μm, so that spurious due to reflection of bulk wave is prevented. Further, Ra of the bonding surface of the supporting body is made 10 nm, so that the bonding strength of the supporting body and filler layer is improved.